The present invention relates to zoom lens systems, and more particularly a zoom lens system suitable for a projection lens for magnifies and projecting an original image formed by an image-forming device, such as a liquid crystal panel. The present invention is suitable, for example, for a zoom lens system including five lens units or more and using a diffraction optical element.
Along with recent increasingly improved fine and miniature liquid crystal projectors, a projection optical system for use with the liquid crystal projectors is required to exhibit higher performance. In particular, a strict requirement has currently applied to a correction to the transverse or lateral chromatic aberration, which dominantly controls the image quality.
Various optical systems have conventionally been proposed as a solution for the above problem. Recently, a diffraction optical element having a diffractive operation has been proposed as a means for correcting the chromatic aberration, in particular, the lateral chromatic aberration for use with the projection optical system.
For example, Japanese Laid-Open Patent Application No. 2000-19400 proposes a zoom lens system that uses a diffraction optical element for the zoom lens system for a projection in a liquid crystal projector. The zoom lens system includes, in order from a more distant conjugate point for a zoom lens system, a first lens unit of negative refractive power, a second lens unit of positive refractive power, a third lens unit of negative refractive power, and a fourth lens unit of positive refractive power, and a fifth lens unit of positive refractive power, and introduces a diffraction optical element into the zoom lens system. This prior art provides, when properly arranging the diffraction optical element in the zoom lens system, a zoom lens system with reduced astigmatism and distortion, which is so compatible with a fine liquid crystal that it may sufficiently correct the lateral chromatic aberration.
However, Japanese Laid-Open Patent Application No. 2000-19400 cannot sufficiently reduce a moving amount of the second unit as a zooming unit from the less distant conjugate point to the more distant conjugate point during zooming from a wide-angle end to a telephoto end, thus is disadvantageous in that the entire lens system does not have a sufficiently reduced size.
Accordingly, it is a primary but exemplified object of the present invention to provide a zoom lens system suitable for a projection lens in a liquid crystal projector etc., which may correct a wide variety of aberrations including the chromatic aberration (in particular, the lateral chromatic aberration), while miniaturizing the entire lens system sufficiently.
In order to achieve the above object, a zoom lens system of one aspect according to the present invention includes, in order from a more distant conjugate point for the zoom lens system, a first lens unit of a negative refractive power, a second lens unit of negative refractive power which moves during zooming, third, fourth and fifth lens units, wherein said zoom lens system further includes at least one diffraction optical element. The zoom lens system is a system for arranging two conjugate points in a conjugate relationship. This zoom lens corrects, when properly arranging the diffraction optical element, a wide variety of aberrations including the chromatic aberration (in particular, lateral the chromatic aberration), while miniaturizing the entire lens system sufficiently.
Preferably, the third, fourth, and fifth lens units have positive, negative and positive refractive power.
The zoom lens system may further include a sixth lens unit, wherein the fifth lens unit moves during the zooming.
The zoom lens system may further include a sixth lens unit of positive refractive power.
The zoom lens system may further include a stop movable during zooming. The zoom lens system may further include stop between the third and forth lens units. The stop may move with the third lens unit. The diffraction optical element may be located closer to a less distant conjugate point than the stop. Thereby, the axial and lateral chromatic aberrations can be simultaneously corrected.
The diffraction optical element may be located in the fifth lens unit, thereby preventing lowered diffraction efficiency caused by an angular difference relative to a tangential direction at each incident position on a lens surface of each of the axial and non-axial rays as well as the deteriorated diffractive optical element at the outermost lens surface due to the dust and heat from a light source.
A conditional expression xe2x88x920.50 less than fw/f1 less than xe2x88x920.01 is preferably satisfied where f1 is a focal length of the first lens unit and fw is a focal length of the entire system at a wide-angle end. In this conditional expression, when the ratio exceeds the lower limit, the negative refractive power of the first lens unit becomes excessively strong, and the curvature of field is undesirably generated at an excessive side. On the other hand, when the ratio exceeds the upper limit, the negative refractive power of the first lens unit becomes excessively weak. Thereby, it becomes difficult to provide the entire system with a short focal length, and undesirably causes a long projection length to project an image.
The fourth lens unit moves from the more distant conjugate point to the less distant conjugate point during zooming from the wide-angle end to the telephoto end.
A conditional expression 0.05 less than d3W/d3T less than 0.60 is preferably satisfied where d3W is a separation between the third and fourth lens units at the wide-angle end and d3T is a separation between the third and fourth lens units at the telephoto end. When the ratio exceeds the upper limit, the entire lens system and thus the back diameter undesirably increase. When the ratio exceeds the lower limit, a position of an exit pupil undesirably drastically fluctuates during zooming.
The second lens unit preferably moves from the more distant conjugate point to the less distant conjugate point during zooming from the wide-angle end to the telephoto end.
A conditional expression 0.05 less than |M2/M4| less than 1.0 is satisfied where M2 is a moving amount of the second lens unit during zooming from the wide-angle end to the telephoto end, and M4 is a moving amount of the fourth lens unit during zooming from the wide-angle end to the telephoto end. In this conditional expression, when the ratio exceeds the upper limit, the front diameter becomes undesirably large. When the ratio exceeds the lower limit, the predetermined zooming ratio cannot be undesirably maintained.
A conditional expression 0.01 less than |M3/M4| less than 1.0 is satisfied where M3 is a moving amount of the third lens unit during zooming from the wide-angle end to the telephoto end, and M4 is a moving amount of the fourth lens unit during zooming from the wide-angle end to the telephoto end. In this conditional expression, when the ratio exceeds the upper limit, the lens span becomes undesirably long. When the ratio exceeds the lower limit, a predetermined zooming ratio cannot be undesirably maintained.
Preferably, the lens units at both ends do not move during zooming from the wide-angle end to the telephoto end.
Preferably, the second lens unit includes one positive lens and one negative lens, or only one negative lens. Preferably, the third lens unit includes one positive lens. Preferably, the fourth lens unit includes one negative lens.
Preferably, the first lens unit includes, in order from the more distant conjugate point, three lenses of a positive lens, a negative lens and a negative lens, or a positive lens, a positive lens and a negative lens.
Preferably, the diffraction optical element includes one diffraction grating so as to enhance the diffraction efficiency of the diffraction light near a designed order or stacked layers of diffraction gratings so as to enhance the optical performance (or diffraction efficiency).
The diffraction optical element is formed by combining two diffraction gratings having the same grating thickness and facing each other so as to make flat a surface of the diffraction optical element. Such a diffraction optical element""s surface as does not form a grating shape facilitates assembly work of the diffraction optical element, and provides a dustproof and inexpensive optical system.
The diffraction optical element may be formed by combining a plurality of diffraction gratings or a plurality of diffraction gratings facing each other via air.
An image-forming device of another aspect of the present invention includes the above zoom lens system and projects an original image to a subject surface, the image being located at the less distant conjugate position for the zoom lens system. The original image is a liquid crystal panel.
An image pick-up device of still another aspect of the present invention includes the above zoom lens, and uses the zoom lens system to project an image of an object onto a photosensitive body located at a less distant conjugate position for the zoom lens system.